Design for Multiple Off-Axis Illumination Beams for Wafer Alignment Sensor

This application claims priority of U.S. application 63/355,220 which was filed on Jun. 24, 2022 and which is incorporated herein in its entirety by reference.

The present disclosure relates to an alignment system that may be used, for example, in a lithographic apparatus.

A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one exposure, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate.

During lithographic operation, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it may be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks, which may comprise diffraction gratings are placed on the substrate to be aligned and are located with reference to a second object. Lithographic apparatus may use an alignment system for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask.

Alignment systems typically have their own illumination system that may be used to illuminate the alignment marks during alignment measurements. Determining alignment typically includes determining the position of an alignment mark (or marks) and/or other target in a layer of a semiconductor device structure. Alignment is typically determined by irradiating an alignment mark with radiation, and comparing characteristics of different diffraction orders of radiation reflected from the alignment mark. Similar techniques are used to measure overlay and/or other parameters. Current alignment sensors have a single measurement illumination spot projected onto a substrate (e.g., a wafer). The single illumination spot is used for measurements of multiple alignment parameters, phase, and intensity detection. Current sensors measure metrology marks serially. Thus, the number of measured marks on a given substrate is limited by throughput considerations.

Alignment can be performed using off-axis illumination beams. Illumination beams are considered off-axis when they are not normal to the substrate. Contrast this to an on-axis illumination beam, which comes down the center of the optical axis normal to the substrate. Off-axis illumination helps in detecting finer pitch alignment marks using the existing objective system. As a result, the process window can be increased for the customer.

Disclosed are novel systems and methods to create off-axis illumination beams. Off-axis illumination using glass plates design if compared to grating-based off-axis illumination results in space savings within the optical system. The use of glass plates for illumination can create colinear beams of different wavelengths, as opposed to the dispersed spectrum of light for grating-based illumination. The glass plates can be finely adjusted, allowing for better control of the off-axis illumination beams.

According to an embodiment, an alignment illumination system includes an illumination source, at least a pair of glass plates, a transmissive optic with at least one reflective mirror configured to reflect radiation received from the glass plates, and an objective configured to focus light from the transmissive optic towards an object to be illuminated. Each glass plate has a plurality of spot mirrors that can reflect or partially reflect and transmit an illumination beam from the illumination source.

In an embodiment, the optical system reflects and refracts the illumination beam to create pairs of off-axis illumination beams. In an embodiment, the off-axis illumination beams in each pair are in phase with each other.

In an embodiment, the optical path differences of the off-axis illumination beams in each pair can be controlled by tilting one glass plate in the pair with respect to the other using a glass plate drive motor.

In an embodiment, the off-axis illumination beams have small color separation or are colinear for different wavelengths. In an embodiment, the illumination angles for all colors of the off-axis illumination beams are the same.

In an embodiment, the spectral content of the off-axis or the on-axis illumination beams can be controlled by adding thin film coatings onto the spot mirrors.

In an embodiment, the off-axis illumination beams are polarized by adding thin film coatings onto the spot mirrors. In an embodiment, the off-axis illumination beams are unpolarized.

In an embodiment, an aperture plate positioned between the glass plates and the transmissive optic blocks unwanted radiation. In an embodiment, the aperture plate blocks an on-axis illumination beam and allows off-axis illumination beams to pass through. In an embodiment, the aperture plate blocks off-axis illumination beams and allows an on-axis illumination beam to pass through. In an embodiment, the aperture plate allows an on-axis illumination beam and off-axis illumination beams to pass through.

In an embodiment, the plurality of spot mirrors comprises metal or dielectric or a combination thereof.

In an embodiment, the at least a pair of glass plates comprises exactly two glass plates. In an embodiment, the two glass plates create a pair of off-axis illumination beams.

In an embodiment, the at least a pair of glass plates comprises four glass plates to create two pairs of off-axis illumination beams.

In an embodiment, an alignment system measuring alignment of an object. An alignment illumination system and an adjustable aperture stop can be inserted into the alignment system to create pairs of off-axis illumination beams.

According to an embodiment, an alignment system includes an illumination source, at least a pair of glass plates, an aperture stop, a transmissive optic with at least one reflective mirror configured to reflect radiation received from the glass plates, an objective configured to focus light from the transmissive optic towards an object to be illuminated, an object stage for holding an object, and an interferometer configured to measure a mark on the object. Each glass plate has a plurality of spot mirrors that can reflect or partially reflect and transmit an illumination beam from the illumination source.

According to an embodiment, a method for alignment using an alignment illumination system includes illuminating an illumination beam from an illumination source, reflecting the illumination beam from the illumination source using at least a pair of glass plates, reflecting radiation received from the glass plates using a transmissive optic with at least one reflective mirror, and focusing light from the transmissive optic towards an object to be illuminated using an objective. Each glass plate has a plurality of spot mirrors.

According to an embodiment, a method for alignment using an alignment illumination system includes illuminating an illumination beam from an illumination source, reflecting or partially reflecting and transmitting the illumination beam from the illumination source using at least a pair of glass plates, filtering radiation received from the glass plates using an aperture stop, reflecting radiation received from the aperture stop using a transmissive optic with at least one reflective mirror, focusing light from the transmissive optic towards an object held by an object stage to be illuminated using an objective, and measuring a mark on the object using an interferometer. Each glass plate has a plurality of spot mirrors.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

1 FIG. 100 109 100 112 109 110 111 117 122 124 126 128 130 112 113 109 109 113 110 113 111 111 113 120 118 shows an alignment systemthat uses a grating structureto create off-axis illumination beams. Alignment systemmay include an illumination system, a grating structure, a lens, a mirror, an objective, a stagemovable along direction, an image rotation interferometer, a detector, and a signal analyzer. The illumination systemgenerates radiation beamthat impinges upon the grating structure. The gratings of the grating structurediffracts radiation beaminto a wide angle of light rays. Subsequently, lensfocuses radiation beamonto mirror. Mirrormay be configured to direct radiation beamonto a substratefor illuminating an alignment mark.

113 118 119 126 121 127 128 120 130 129 122 122 118 Radiation beamthen reflects off the alignment markto become diffracted radiation beam, which is directed toward image rotation interferometeralong alignment axis. Thereafter, signalis passed to detectorto detect a position of the substrate. Signal analyzerthen receives signalto determine a position of the stageand correlate the position of the stagewith the position of the center of symmetry of alignment mark.

An issue with this grating structure alignment system is that since the grating structure diffracts the radiation beam into a wide angle of light rays, the radiation beam covers a significant amount of space. Preserving all of the beam, which includes chromatic light at different angles, requires a large amount of volume that is dedicated to the transmission of the light. As a result, the grating alignment system is unwieldy and cumbersome.

1 FIG. In contrast to the system shown in, the present alignment system uses glass plates to create off-axis illumination beams. As few as two beams can be used, which is significantly less than for gratings. Reducing the amount of beams from a wide beam used in grating alignment to only two beams in glass plate alignment, much space can be saved within the optical system. As a result, the footprint of the optical system can be greatly reduced. Further, other advantages of using glass plates for alignment include all wavelengths being colinear, as opposed to the dispersed spectrum of light for grating alignment. The glass plates can be finely adjusted, allowing for better control of the off-axis illumination beams. Another benefit of off-axis alignment using glass plates is in efficiency. Only the needed number of off-axis illumination beams is created, while grating-based illumination would generally have additional diffraction orders that have to be blocked or otherwise removed from the system.

Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

2 2 FIGS.A andB 200 200 200 200 are schematic illustrations of a lithographic apparatusand lithographic apparatus′, in or for which embodiments of the present disclosure may be implemented. Lithographic apparatusand lithographic apparatus′ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer)

200 200 200 200 W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatusand′ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus, the patterning device MA and the projection system PS are reflective. In lithographic apparatus′, the patterning device MA and the projection system PS are transmissive.

200 200 The illumination system IL may include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B. The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatusand′, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

200 200 2 FIG.B 2 FIG.A The patterning device MA may be transmissive (as in lithographic apparatus′ of) or reflective (as in lithographic apparatusof). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B which is reflected by a matrix of small mirrors.

The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

200 200 Lithographic apparatusand/or lithographic apparatus′ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

2 2 FIGS.A andB 2 FIG.B 200 200 200 200 200 200 Referring to, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus,′ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatusor′, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus,′—for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if required, can be referred to as a radiation system.

2 FIG.B 2 FIG.B The illuminator IL can include an adjuster AD (in) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “o-outer” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

2 FIG.A 200 2 1 1 2 1 2 Referring to, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF(for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IFcan be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks M, Mand substrate alignment marks P, P.

2 FIG.B Referring to, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil PPU conjugate to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at a mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

2 FIG.B With the aid of the second positioner PW and position sensor IF (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

1 2 1 2 In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks M, M, and substrate alignment marks P, P. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

Mask table MT and patterning device MA can be in a vacuum chamber, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

200 200 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de-) magnification and image reversal characteristics of the projection system PS. 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array. The lithographic apparatusand′ can be used in at least one of the following modes:

Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

200 In some embodiments, lithographic apparatusincludes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

2 FIG.C 200 220 210 210 210 shows the lithographic apparatusin more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structureof the source collector apparatus SO. An EUV radiation emitting plasmamay be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the hot plasmais created to emit radiation in the EUV range of the electromagnetic spectrum. The hot plasmais created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

210 211 212 230 211 230 230 230 The radiation emitted by the hot plasmais passed from a source chamberinto a collector chambervia an optional gas barrier or contaminant trap(in some cases also referred to as contaminant barrier or foil trap) which is positioned in or behind an opening in source chamber. The contaminant trapmay include a channel structure. Contamination trapmay also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrierfurther indicated herein at least includes a channel structure, as known in the art.

212 251 252 240 219 220 210 240 The collector chambermay include a radiation collector CO which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector sideand a downstream radiation collector side. Radiation that traverses collector CO can be reflected off a grating spectral filterto be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus IF is located at or near an openingin the enclosing structure. The virtual source point IF is an image of the radiation emitting plasma. Grating spectral filteris used in particular for suppressing infra-red (IR) radiation.

222 224 221 221 226 226 228 230 Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror deviceand a facetted pupil mirror devicearranged to provide a desired angular distribution of the radiation beam, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiationat the patterning device MA, held by the support structure MT, a patterned beamis formed and the patterned beamis imaged by the projection system PS via reflective elements,onto a substrate W held by the wafer stage or substrate table WT.

240 2 FIG.C More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filtermay optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the FIGS., for example there may be 1-6 additional reflective elements present in the projection system PS than shown in.

2 FIG.C 253 254 255 253 254 255 Collector optic CO, as illustrated in, is depicted as a nested collector with grazing incidence reflectors,and, just as an example of a collector (or collector mirror). The grazing incidence reflectors,andare disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

3 FIG.A 300 200 200 300 300 1 2 shows a lithographic cell, also sometimes referred to a lithocell or cluster. Lithographic apparatusor′ may form part of lithographic cell. Lithographic cellmay also include apparatus to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/O, I/O, moves them between the different process apparatus and delivers then to the loading bay LB of the lithographic apparatus. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency.

One or more targets are specifically provided on the substrate to enable alignment. Typically, the target is specially designed and may comprise a periodic structure. For example, the target on a substrate may comprise one or more 1-D periodic structures (e.g., geometric features such as gratings), which are printed such that after development, the periodic structural features are formed of solid resist lines. As another example, the target may comprise one or more 2-D periodic structures (e.g., gratings), which are printed such that after development, the one or more periodic structures are formed of solid resist pillars or vias in the resist. The bars, pillars, or vias may alternatively be etched into the substrate (e.g., into one or more layers on the substrate).

3 FIG.B 3 FIG.C 3 FIG.C 10 2 4 depicts an example alignment systemthat may be used to detect alignment as well as overlay and/or perform other metrology operations. It comprises a radiation or illumination sourcewhich projects or otherwise irradiates radiation onto a substrate W (e.g., which may typically include a metrology mark). The redirected radiation is passed to a sensor such as a spectrometer detectorand/or other sensors, which measures a spectrum (intensity as a function of wavelength) of the specular reflected and/or diffracted radiation, as shown, e.g., in the graph on the left of. The sensor may generate an alignment signal conveying alignment data indicative of properties of the reflected radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by one or more processors PRO, a generalized example of which is shown in, or by other operations.

200 200 10 2 2 FIGS.A andB 2 2 FIGS.A andB As in the lithographic apparatusand′ of, one or more substrate tables may be provided to hold the substrate W during measurement operations. The one or more substrate tables may be similar or identical in form to the substrate table WT of. In an example where inspection systemis integrated with the lithographic apparatus, they may even be the same substrate table. Coarse and fine positioners may be provided and configured to accurately position the substrate in relation to a measurement optical system. Various sensors and actuators are provided, for example, to acquire the position of a target portion of interest of a structure (e.g., a metrology mark), and to bring it into position under an objective lens. Typically, many measurements will be made on target portions of a structure at different locations across the substrate W. The substrate support can be moved in X and Y directions to acquire different targets, and in the Z direction to obtain a desired location of the target portion relative to the focus of the optical system. It is convenient to think and describe operations as if the objective lens is being brought to different locations relative to the substrate, when, for example, in practice the optical system may remain substantially stationary (typically in the X and Y directions, but perhaps also in the Z direction) and the substrate moves. Provided the relative position of the substrate and the optical system is correct, it does not matter in principle which one of those is moving, or if both are moving, or a combination of a part of the optical system is moving (e.g., in the Z and/or tilt direction) with the remainder of the optical system being stationary and the substrate is moving (e.g., in the X and Y directions, but also optionally in the Z and/or tilt direction).

30 30 For typical alignment measurements, a target (portion)on substrate W may be a 1-D grating, which is printed such that after development, the bars are formed of solid resist lines (e.g., which may be covered by a deposition layer), and/or other materials. Or the targetmay be a 2-D grating, which is printed such that after development, the grating is formed of solid resist pillars, and/or other features in the resist.

30 30 30 The bars, pillars, vias, and/or other features may be etched into or on the substrate (e.g., into one or more layers on the substrate), deposited on a substrate, covered by a deposition layer, and/or have other properties. Target (portion)(e.g., of bars, pillars, vias, etc.) is sensitive to changes in processing in the patterning process (e.g., optical aberration in the lithographic projection apparatus such as in the projection system, focus change, dose change, etc.) such that process variation manifests in variation in target. Accordingly, the measured data from targetmay be used to determine an adjustment for one or more of the manufacturing processes, and/or used as a basis for making the actual adjustment.

30 30 For example, the measured data from targetmay indicate overlay for a layer of a semiconductor device. The measured data from targetmay be used (e.g., by the one or more processors PRO and/or other processors) for determining one or more semiconductor device manufacturing process parameters based on the overlay, and determining an adjustment for a semiconductor device manufacturing apparatus based on the one or more determined semiconductor device manufacturing process parameters. In some embodiments, this may comprise a stage position adjustment, for example, or this may include determining an adjustment for a mask design, a metrology target design, a semiconductor device design, an intensity of the radiation, an incident angle of the radiation, a wavelength of the radiation, a pupil size and/or shape, a resist material, and/or other process parameters.

3 FIG.D 5 FIG. 30 30 illustrates a plan view of a typical target (e.g., metrology mark), and the extent of a typical radiation illumination spot S in the system of. Typically, to obtain a diffraction spectrum that is free of interference from surrounding structures, the target, in an embodiment, is a periodic structure (e.g., grating) larger than the width (e.g., diameter) of the illumination spot S. The width of spot S may be smaller than the width and length of the target. The target, in other words, is ‘underfilled’ by the illumination, and the diffraction signal is essentially free from any signals from product features and the like outside the target itself. The illumination arrangement may be configured to provide illumination of a uniform intensity across a back focal plane of an objective, for example. Alternatively, by, for example, including an aperture in the illumination path, illumination may be restricted to on-axis or off-axis directions.

4 FIG. 400 200 200 400 400 100 100 illustrates a schematic of a cross-sectional view of an alignment systemthat can be implemented as a part of or otherwise in conjunction with lithographic apparatusor′, and/or other lithographic apparatuses, according to an embodiment. In an example of this embodiment, alignment systemmay be configured to align a substrate (e.g., a semiconductor wafer, substrate W described above, etc.) with respect to a patterning device (e.g., patterning device MA described above). Alignment systemmay be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithography apparatusor′ using the detected positions of the alignment marks. Such alignment of the substrate may ensure accurate exposure of one or more patterns on the substrate.

400 412 414 417 426 428 430 412 413 413 413 413 412 412 413 According to an embodiment, alignment systemmay include an illumination source, an optical system, an objective, an image rotation interferometer, a detector, and a signal analyzer. Illumination sourcemay be configured to provide an electromagnetic narrow band radiation beamhaving a first polarization state, such as a linear polarization state. In an example, the narrow band radiation beammay be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the narrow band radiation beamcomprises discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Yet in another example, radiation beammay be monochromatic, for example, provided by a monochromatic light source, such as a laser light source in illumination source. Polychromatic light sources such as LEDs may also be used in illumination sourceto provide a polychromatic radiation beam.

414 413 413 420 414 413 418 420 414 413 418 4 FIG. Optical systemmay be configured to receive radiation beam. In an example of this embodiment, optical system may be further configured to direct radiation beamonto a substrate, as shown in. Optical systemmay include a transmissive optic with at least one reflective mirror configured to direct radiation beamtowards alignment marklocated on substrate. Optical systemmay also include optics that create additional illumination beams split or replicated from radiation beamand direct them towards alignment mark.

420 422 424 413 418 420 418 418 418 418 418 418 Substratemay be placed on a stagemoveable along direction. Radiation beammay be configured to illuminate an alignment marklocated on substrate. Alignment markmay be coated with a radiation sensitive film in an example of this embodiment. In another example, alignment markmay have 180° symmetry. That is, when alignment markis rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark, rotated alignment markmay be substantially identical to an unrotated alignment mark.

4 FIG. 4 FIG. 417 419 426 417 419 419 413 418 419 414 414 419 419 419 417 419 426 418 As illustrated in, objectivemay be configured to direct diffracted radiation beamtowards image rotation interferometer, according to an embodiment. Objectivemay comprise any appropriate number of optical elements suitable for directing diffracted radiation beam. In an example embodiment, diffracted radiation beammay be at least a portion of radiation beamthat is diffracted from alignment mark. It should be noted that even though diffracted radiation beamhas been shown to pass outside optical systemin, the disclosure in not so limiting. Optical systemmay be substantially transparent to diffracted radiation beamand may allow diffracted radiation beamto pass through it without substantially changing the properties of diffracted radiation beam. It should be further noted that even though objectiveis shown to direct radiation beamtowards image rotation interferometer, the disclosure is not so limiting. Other optical arrangements may be used to obtain the similar result of detecting diffraction signals from alignment mark.

426 418 419 418 426 In some embodiments, image rotation interferometermay comprise any appropriate set of optical elements, for example, a combination of prisms that may be configured to form two images of alignment markbased on the received diffracted radiation beam. It should be appreciated that a good quality image need not be formed, but that the features of alignment markshould be resolved. Image rotation interferometermay be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

428 421 400 418 418 428 418 420 421 420 426 Detectormay be configured to receive the recombined image and detect an interference as a result of the recombined image when alignment axisof alignment systempasses through a center of symmetry (not shown) of alignment mark. Such interference may be due to alignment markbeing 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example embodiment. Based on the detected interference, detectormay be further configured to determine a position of the center of symmetry of alignment markand consequently, detect a position of substrate. According to an example, alignment axismay be aligned with an optical beam perpendicular to substrateand passing through a center of image rotation interferometer.

430 429 430 422 422 418 418 420 422 430 400 418 400 In some embodiments, signal analyzermay be configured to receive signalincluding information of the determined center of symmetry. Signal analyzermay be further configured to determine a position of stageand correlate the position of stagewith the position of the center of symmetry of alignment mark. As such, the position of alignment markand consequently, the position of substratemay be accurately known with reference to stage. Alternatively, signal analyzermay be configured to determine a position of alignment systemor any other reference element such that the center of symmetry of alignment markmay be known with reference to alignment systemor any other reference element.

5 FIG. 4 FIG. 4 FIG. 514 514 414 513 413 513 510 illustrates a schematic of a cross-sectional view of an optical system, according to an embodiment. Optical systemmay represent an example embodiment of optical systemshown in. Illumination beammay be similar to radiation beamdiscussed above with respect to. Illumination beamis on-axis since it travels along optical axis.

514 550 552 553 550 560 513 513 550 515 515 550 552 513 515 552 553 553 563 563 563 513 515 417 417 513 515 420 513 515 418 420 5 FIG. 4 FIG. In an example of this embodiment, optical systemmay comprise pair of glass plates, aperture plate, and transmissive optic. Glass plateseach have a plurality of spot mirrorsfor reflecting and refracting illumination beam. As a result, illumination beamis incident on glass platesto create a pair of illumination beams. Illumination beamsare off-axis. Downstream of glass platesis aperture plate, which selectively blocks unwanted radiation.shows the case where on-axis illumination beamand off-axis illumination beamsare preserved. Downstream of aperture plateis transmissive optic. Transmissive opticcomprises a plurality of reflective mirrors. Each reflective mirroris associated with an illumination beam. Reflective mirrorsreflect on-axis illumination beamand off-axis illumination beamstoward objective. Objectivemay be configured to focus illumination beamsandonto substrate. Illumination beamsandconverge on alignment mark, shown in, of substrate.

6 8 FIGS.- 6 FIG. 7 FIG. 8 FIG. 4 FIG. 4 FIG. 614 614 614 414 613 413 613 610 illustrate a schematic of a cross-sectional view of optical system, according to an embodiment.is a top view,is a side view, andis a rotated view of optical system. Optical systemmay represent an example embodiment of optical systemshown in. Illumination beammay be similar to radiation beamdiscussed above with respect to. Illumination beamis on-axis since it travels along optical axis.

614 650 651 652 653 650 651 660 661 613 613 650 615 615 615 615 615 613 613 651 616 616 615 616 650 651 652 613 615 616 652 653 653 663 663 663 613 615 616 417 417 613 615 616 420 613 615 616 418 420 6 8 FIGS.and 7 FIG. 7 8 FIGS.and 6 FIG. 6 8 FIGS.- 7 8 FIGS.and 7 8 FIGS.and 4 FIG. In an example of this embodiment, optical systemmay comprise first pair of glass plates, second pair of glass plates, aperture plate, and transmissive optic. Glass platesandeach have a plurality of spot mirrorsand, respectively, for reflecting and refracting radiation beam. As a result, illumination beamis incident on first pair of glass platesto create first pair of illumination beams, shown clearly in.only shows first illumination beam, because second beamis underneath and covered by first beamfrom this view (first beamalso covers illumination beam). Further, illumination beamis then incident on second pair of glass platesto create second pair of illumination beams, shown clearly in. Similar to above,only shows first illumination beam. Illumination beamsandare off-axis illumination beams. Downstream of glass platesandis aperture plate, which selectively blocks unwanted radiation.show the case where on-axis illumination beamand off-axis illumination beamsandis preserved. Downstream of aperture plateis transmissive optic. Transmissive opticcomprises a plurality of reflective mirrors. Each reflective mirroris associated with an illumination beam. Reflective mirrorsreflect on-axis illumination beamand off-axis illumination beamsandtoward objective, shown in. Objectivemay be configured to focus illumination beams,, andonto substrate, shown in. Illumination beams,, andconverge on alignment mark, shown in, of substrate.

While the embodiments described above have either one or two pairs of off-axis illumination beams, the system is not so limited. It is possible to create more than two pairs of off-axis illumination beams.

550 650 651 550 650 651 513 613 515 615 616 Glass plates,, andare each a pair of glass plates. Each pair of glass plates,, andreceives illumination beamandto create a pair of off-axis illumination beams,, and, respectively. Additional pairs of glass plates can be inserted into the optical path to create additional pairs of off-axis illumination beams. Further, one glass plate in the pair can be finely tilted with respect to the other to change characteristics of the off-axis illumination beams. The glass plates can be tilted using a glass plate drive motor (not shown). As a result, optical path differences, amount of light, and power among the pair of beams can be controlled.

560 660 661 The plurality of spot mirrors,, andare partially or fully reflective spot mirrors. Partially reflective spot mirrors refract some of the radiation beam, while reflecting the remainder of the beam. On the other hand, fully reflective spot mirrors reflect the entirety of the beam. Each spot mirror is a discrete area of reflective coating on the glass plate. The reflective coating comprises metal or dielectric or a combination thereof.

552 652 550 650 651 553 653 552 652 418 552 652 552 652 513 613 515 615 616 552 652 515 615 616 513 613 552 652 513 613 515 615 616 Aperture plateandis positioned between the glass plates,, andand the transmissive opticand. Aperture plateandblocks unwanted radiation while allowing certain illumination beams to pass through and reach alignment mark. Aperture plateandis movable into and out of the path of the illumination beams using an aperture plate drive motor (not shown). In a first configuration, aperture plateandblocks an on-axis illumination beamandand allows off-axis illumination beams,, andto pass through. In a second configuration, aperture plateandblocks off-axis illumination beams,, andand allows an on-axis illumination beamandto pass through. In a third configuration, aperture plateandallows an on-axis illumination beamandand off-axis illumination beams,, andto pass through.

553 653 563 563 553 653 417 Transmissive opticandcomprises a plurality of reflective mirrors. Reflective mirrorscan be fully reflective. The number of mirrors in the transmissive optic is equal to the number of illumination beams. Transmissive opticandreflects each of the beams toward objective.

515 615 616 515 615 616 550 650 651 560 660 661 Further details of the pairs of off-axis illumination beams follow. Pairs of off-axis illumination beams,, andare in phase with each other with a matched optical path. Moreover, optical path differences between beams,, andwithin each pair can be finely adjusted by tilting one glass plate,, and, respectively, with respect to the other glass plate in the pair. Additionally, off-axis illumination beams can be polarized or unpolarized. This can be achieved by adding thin film coatings onto spot mirrors,, and. Alternatively, a polarizer (not shown) in the optical path can change the off-axis illumination beams to either be polarized or unpolarized. Since the off-axis illumination beams are created by glass plates (as opposed to a grating structure), the off-axis illumination beams can have smaller color separation or be made fully achromatic or are colinear for different wavelengths. In other words, the off-axis illumination beams are all the same color and not a spectrum of colors as is for a grating structure. As a result, the illumination angle for all colors of the off-axis illumination beams is the same. Further, the spectral content of the off-axis illumination beams can be controlled by adding film coatings. Film coatings will change the power intensity of the off-axis illumination beams.

514 614 400 514 614 4 FIG. In some embodiments, optical systemandcan be inserted into existing alignment systems, similar to alignment systemin. As a result, optical systemandis backwards compatible and can be used to upgrade existing alignment systems.

an illumination source; at least a pair of glass plates, wherein each glass plate has a plurality of spot mirrors that can reflect or partially reflect and transmit an illumination beam from the illumination source; a transmissive optic with at least one reflective mirror configured to reflect radiation received from the glass plates; and an objective configured to focus light from the transmissive optic towards an object to be illuminated. 1. An alignment illumination system, comprising: 2. The alignment illumination system of clause 1, wherein the glass plates reflect and refract the illumination beam to create pairs of off-axis illumination beams. 3. The alignment illumination system of clause 2, wherein the off-axis illumination beams in each pair are in phase with each other. 4. The alignment illumination system of clause 2, wherein the optical path differences of the off-axis illumination beams in each pair can be controlled by tilting one glass plate in the pair with respect to the other using a glass plate drive motor. 5. The alignment illumination system of clause 2, wherein the off-axis illumination beams have small color separation or are colinear for different wavelengths. 6. The alignment illumination system of clause 2, wherein the illumination angles for all colors of the off-axis illumination beams are the same. 7. The alignment illumination system of clause 2, wherein the spectral content of the off-axis illumination beams or on-axis illumination beam can be controlled by adding thin film coatings onto the spot mirrors. 8. The alignment illumination system of clause 2, wherein the off-axis illumination beams are polarized by adding thin film coatings onto the spot mirrors. 9. The alignment illumination system of clause 2, wherein the off-axis illumination beams are unpolarized. 10. The alignment illumination system of clause 1, wherein an aperture plate positioned between the glass plates and the transmissive optic blocks unwanted radiation. 11. The alignment illumination system of clause 10, wherein the aperture plate blocks an on-axis illumination beam and allows off-axis illumination beams to pass through. 12. The alignment illumination system of clause 10, wherein the aperture plate blocks off-axis illumination beams and allows an on-axis illumination beam to pass through. 13. The alignment illumination system of clause 10, wherein the aperture plate allows an on-axis illumination beam and off-axis illumination beams to pass through. 14. The alignment illumination system of clause 1, wherein the plurality of spot mirrors comprises metal or dielectric or a combination thereof. 15. The alignment illumination system of clause 1, wherein the at least a pair of glass plates comprises exactly two glass plates. 16. The alignment illumination system of clause 15, wherein the two glass plates create a pair of off-axis illumination beams. 17. The alignment illumination system of clause 1, wherein the at least a pair of glass plates comprises four glass plates to create two pairs of off-axis illumination beams. 18. An alignment system for measuring alignment of an object, wherein the alignment illumination system of clause 1 and an adjustable aperture stop can be inserted into the alignment system to create pairs of off-axis illumination beams. an illumination source; at least a pair of glass plates, wherein each glass plate has a plurality of spot mirrors that can reflect or partially reflect and transmit an illumination beam from the illumination source; an aperture stop; a transmissive optic with at least one reflective mirror configured to reflect radiation received from the glass plates; an objective configured to focus light from the transmissive optic towards an object to be illuminated; an object stage for holding the object to be illuminated; and an interferometer configured to measure a mark on the object. 19. An alignment system, comprising: illuminating—an illumination beam from an illumination source; reflecting or partially reflecting and transmitting the illumination beam from the illumination source using at least a pair of glass plates, wherein each glass plate has a plurality of spot mirrors; reflecting radiation received from the glass plates using a transmissive optic with at least one reflective mirror; and focusing light from the transmissive optic towards an object to be illuminated using an objective. 20. A method for alignment using an alignment illumination system, comprising: 21. The method of clause 20, wherein the glass plates reflect and refract the illumination beam to create pairs of off-axis illumination beams. 22. The method of clause 21, wherein the off-axis illumination beams in each pair are in phase with each other. 23. The method of clause 21, wherein the optical path differences of the off-axis illumination beams in each pair can be controlled by tilting one glass plate in the pair with respect to the other using a glass plate drive moto 24. The method of clause 21, wherein the off-axis illumination beams have small color separation or are colinear for different wavelengths. 25. The method of clause 21, wherein the illumination angles for all colors of the off-axis illumination beams are the same. 26. The method of clause 21, wherein the spectral content of the off-axis illumination beams or on-axis illumination beam can be controlled by adding thin film coatings onto the spot mirrors. 27. The method of clause 21, wherein the off-axis illumination beams are polarized by adding thin film coatings onto the spot mirrors. 28. The method of clause 21, wherein the off-axis illumination beams are unpolarized. 29. The method of clause 20, wherein an aperture plate positioned between the glass plates and the transmissive optic blocks unwanted radiation. 30. The method of clause 29, wherein the aperture plate blocks an on-axis illumination beam and allows off-axis illumination beams to pass through. 31. The method of clause 29, wherein the aperture plate blocks off-axis illumination beams and allows an on-axis illumination beam to pass through. 32. The method of clause 29, wherein the aperture plate allows an on-axis illumination beam and off-axis illumination beams to pass through. 33. The method of clause 20, wherein the plurality of spot mirrors comprises metal or dielectric or a combination thereof. 34. The method of clause 20, wherein the at least a pair of glass plates comprises exactly two glass plates. 35. The method of clause 34, wherein the two glass plates create a pair of off-axis illumination beams. 36. The method of clause 20, wherein the at least a pair of glass plates comprises four glass plates to create two pairs of off-axis illumination beams. 37. A method for measuring alignment of an object using an alignment system, wherein the alignment illumination system of clause 20 and an adjustable aperture stop can be inserted into the alignment system to create pairs of off-axis illumination beams. illuminating an illumination beam from an illumination source; reflecting or partially reflecting and transmitting the illumination beam from the illumination source using at least a pair of glass plates, wherein each glass plate has a plurality of spot mirrors; filtering radiation received from the glass plates using an aperture stop; reflecting radiation received from the aperture stop using a transmissive optic with at least one reflective mirror; focusing light from the transmissive optic towards an object held by an object stage to be illuminated using an objective; and measuring a mark on the object using an interferometer. 38. A method for alignment using an alignment illumination system, comprising: The embodiments may further be described using the following clauses:

This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto. The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

2 Further, the terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (for example, having a wavelengthof 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in an embodiment, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

The term “substrate” as used herein generally describes a material onto which subsequent material layers are added. In embodiments, the substrate itself may be patterned and materials added on top of it may also be patterned, or may remain without patterning.

The term “in substantial contact” as used herein generally describes elements or structures that are in physical contact with each other with only a slight separation from each other which typically results from misalignment tolerances. It should be understood that relative spatial descriptions between one or more particular features, structures, or characteristics (e.g., “vertically aligned,” “substantial contact,” etc.) used herein are for purposes of illustration only, and that practical implementations of the structures described herein may include misalignment tolerances without departing from the spirit and scope of the present disclosure.

The term “optically coupled” as used herein generally refers to one coupled element being configured to impart light to another coupled element directly or indirectly.

The term “optical material” as used herein generally refers to a material that allows light or optical energy to propagate therein or therethrough.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.

It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.

The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments but should be defined only in accordance with the following claims and their equivalents.